Utilizing clear solar energy to generate electricity is one of the most effective means for solving the problem of current energy shortage. However, since output voltage and output current of photovoltaic array are influenced by external environment (e.g., illumination intensity, temperature, load characteristic) and factors of itself, output characteristics of photovoltaic array demonstrate obvious non-linear characteristics, with which output power thereof varies constantly. At certain illumination intensity and certain temperature, voltage-power output characteristic curve in which output power of the photovoltaic array changes with operating voltage is a single-peak curve. The photovoltaic array can output the maximum power only when it operates at the peak of the curve, which is called Maximum Power Point (MPP). Since MPP changes according to the illumination intensity and the temperature, the key to improve overall efficiency of the photovoltaic power generation system is to adjust the operating point of the photovoltaic array in real time to keep near to MPP. This procedure is Maximum Power Point Tracking (MPPT).
Thus, in controlling the photovoltaic power generation system, it is particularly important to adjust the load characteristic so that the photovoltaic power generation system can output maximum power in real time as far as it could so as to achieve MPPT. Typical direct control algorithms include Perturbation & Observation Method (P&O), Conductance Increment Method, and so on. This type of control algorithms has higher accuracy than indirect control methods, can achieve real-time MPPT control, and can satisfy ordinary needs for general applications, thus is most widely used in practical engineering.
P&O is the most commonly used algorithm among the control strategies of MPPT, and it is crucial to select a perturbation step for P&O. If a larger perturbation step is used, response of the system to external condition changes is fast, but power fluctuation near MPP is large; on the contrary, if a smaller perturbation step is used, power fluctuation near MPP is small, but response of the system to external condition changes is slow. Therefore, there are contradictions between the dynamic performance and the static performance of P&O with fixed step.